1. Field of the Invention
This invention relates to a subsurface valve for controlling flow in a well and having a flapper closure element. One form of the valve is surface controlled while other forms open and close depending upon flow and/or pressure conditions at the valve.
2. The Prior Art
Subsurface valves having a flapper valve member are illustrated in the "COMPOSITE CATALOG OF OILFIELD EQUIPMENT AND SERVICES", on pages 479, 1175, 4573, 4574 and 4575 of the 1976-77 edition and pages 4000, 4003, 4008, and 4009 of the 1974-75 edition. The subsurface valves therein disclosed have a common limitation. They all have a rigid, stationary hinge for the flapper closure element. During operation of the respective valves, the hinge of each is subjected to destructive forces. Therefore, upon repeated openings and closings, the rigid, stationary hinge either breaks or is stressed beyond its elastic limit and takes a plastic set. The subsurface valve is thereby rendered inoperative.
Forces which are destructive to the flapper hinge are presently generated during closure for all types of subsurface valves having a flapper closure element and may be generated during the opening of a surface controlled subsurface safety valve having a flapper closure element. Presently, subsurface valves having a flapper closure element also have a control tube. When the valve is open, the control tube extends across the flapper closure element and maintains the closure element out of the flow path. The control tube has to be moved to an out-of-the-way position during valve closure. While the control tube is moving to its out-of-the-way position, the flapper closure element is pivoting towards its on-seat position. During its pivotal movement, the flapper closure element will enter the subsurface flow path. Thereafter, flowing well fluids will impinge upon the flapper closure element and increase the speed at which it is pivoted towards its on-seat position. The control tube cannot move to its out-of-the-way position as rapidly as the flapper closure element is pivoted towards its seat. Therefore, the flapper closure element strikes the control tube violently. When the flapper closure element strikes the control tube, an impactive potentially destructive force is imparted to the flapper hinge. During the opening of a surface controlled subsurface safety valve having a flapper closure, the lower end of the control tube strikes the flapper closure element while the closure element is in its on-seat position. If high shut-in formation pressures exist, the flapper closure element is subjected to large opposing forces. A force generated by the shut-in formation fluids tends to maintain the flapper closure element in its on-seat position. A force applied by the control tube tends to pivot the flapper closure element off of its on-seat position. When the control tube does pivot the flapper closure element off of the seat, the combination of the high opposing forces acting upon the flapper closure element and fluids flowing past the flapper closure element may adversely affect the flapper hinge.
Heretofore, pressure differential operated subsurface valves having a flapper closure element have not been able to quickly close the subsurface flow path. Additionally, such valves have tended to create a motor effect during closure. The inability to quickly close the flow path is due to the use of a control tube. The control tube has to be moved through a relatively long stroke to permit the flapper closure element to move to its on-seat position. Movement of the control tube takes time. Movement of the flapper closure element to its on-seat position is therefore delayed until the control tube has moved to its out-of-the-way position. A motor effect during closure results due to the interaction between the resilient urging opening force and the pressure differential closing force. For a normally opened, pressure differential operated, subsurface valve having a flapper closure element, an inherently resilient means urges the flapper closure element towards its open position. A high rate of fluid flow through the valve creates a differential pressure force which force tends to urge the flapper closure element towards its on-seat position. When the differential pressure force becomes greater than the resilient urging force, valve closure is initiated. However, when the flapper closure element enters the flow path, the flow rate is decreased. In fact, occasionally the fluid flow actually ceases and the fluid "bounces". When that occurs, the shut-in pressure of well fluids below the valve reduces. The resilient urging force tending to open the valve becomes greater than the differential pressure force tending to close the valve. The flapper closure element is moved towards its pivoted open position. The sequence is repeated rapidly causing the flapper closure element to motor.
U.S. Pat. No. 3,072,141 to Wheeler, Jr., discloses a valve having a floating pivotal mounting for the flapper closure element. The hinge of the flapper closure element receives a relatively rigid hinge pin. The patent discloses that either the journals for the hinge pin or the hinge holes may be elongated to provide the floating pivotal mounting.
U.S. Pat. No. 3,981,358 to Watkins et al discloses a valve having a flapper closure element and a rigid flapper hinge. The control tube for moving the flapper closure element comprises at least two telescopable members. The members are designed to telescope to a collapsed position before excessive forces are applied to the control tube or the flapper closure element. Therefore, if such a valve was surface controlled, it could not open if a substantial pressure differential existed across the flapper closure element. Additionally, such a valve would continue to have the deficiencies noted for other valves during closure.